Technology that transmits power without a power line, so-called wireless power transmission technology, has attracted attention. The wireless power transmission technology can transmit power from power feeding device to power receiving device with no contact. So it is expected to be applicable in transporting machines such as trains and electric vehicles, household appliances, electronic devices, wireless communication devices, toys and the like products.
In the wireless power transmission device, the primary coil constituting the power feeding portion of the power feeding device and the secondary coil constituting the power receiving portion of the power receiving device are opposite to each other to supply power from power feeding device to power receiving device. The primary coil and the secondary coil are mostly configured at specific portions of the power feeding device and the power receiving device, and the specific portions of the power feeding device and the power receiving device need to be opposite to each other when charging the power receiving device.
From the viewpoint of configuration of the power receiving device, Patent Document 1 discloses a power feeding system in which a plurality of power feeding side coils and power receiving side coils are provided on the power feeding device and the power receiving device, respectively, such that the user can optionally configure these devices regardless of the positional relation between the power feeding device and the power receiving device. In the aforementioned power feeding system, the circuit of the output signal is arranged such that the power feeding side coil and the power receiving side coil are operated at the highest transmission efficiency among the plurality of power feeding side coils and power receiving side coils.
Furthermore, from the viewpoint of power transmission efficiency, Patent Document 2 discloses a portable device and a bottom surface-curved charger in which a curved induction coil is built-in and the portable device is back surface-curved to follow the shape of the induction coil so that the power can be easily carried from the charger to the portable device.